1. Technical Field
The present invention relates to a screen, a rear projector, a projection system, and an image display unit.
2. Related Art
In recent years, there has been a rapid increase in the popularity of projectors. In addition to front projection projectors that have become widely used principally in business presentation applications, recently, the level of recognition of rear projectors as a type of large screen television (i.e., PTV; projection television) has been increasing. The greatest advantage of the projection format is that, compared with direct view displays such as liquid crystal televisions and PDP and the like, products having the same screen size can be provided at a lower cost. However, costs are also coming down in direct view displays as well and a higher image quality is also being demanded from projector display units.
Projectors display an image on a screen by irradiating light that is emitted from a light source such as an arc lamp onto optical modulators of liquid crystal light bulbs, and then projecting projection light that has been modulated by the optical modulators onto a screen. At this time, not only are images displayed on the screen, but the entire screen appears to glare. This is because of luminance unevenness that is created by the interference of the light rays. This phenomenon is known as speckle noise, or ‘scintillation’.
Here, the principle behind the occurrence of scintillation will be described.
As shown in FIGS. 29A and 29B, light irradiated from a light source 70 is transmitted through a liquid crystal light bulb and is projected onto a screen 74. The projection light that has been projected onto the screen 74 is diffracted by scattering materials 72 that are contained in the screen 74, and is diffused as a result of the scattering materials 72 behaving in the manner of a secondary wave source. As shown in FIG. 29B, two spherical waves that are formed by the secondary wave sources either intensify or offset each other in accordance with their relative phase relationship so as to appear as a light and dark fringe pattern (i.e., interference fringes) between the screen 74 and a viewer. If the focal point of a viewer is matched to an image forming surface S of these interference fringes, then the viewer would notice the interference fringes as scintillation that causes the screen to glare.
Depending on the viewer watching images formed on a screen surface, scintillation may cause an unpleasant view just as if a veil, or lace, or cobweb exists between the screen surface and the viewer. Moreover, the viewer views two overlapping images, namely, the images on the screen and the scintillation, and this creates considerable fatigue in an attempt to focus both the images. Accordingly, this scintillation ends up imparting considerable stress to a viewer which makes viewing almost unendurable.
Recently, advances have been made in the development of new light sources to replace known high pressure mercury lamps. In particular, expectations for laser light sources to become the light sources for next-generation projectors have heightened in view of their energy efficiency, color reproducibility, long life, immediate turning on, and the like. However, projection light on a screen from a laser light source has extremely high interference as a result of the uniformity of the phases of light rays from adjacent areas. Because the coherent length of laser light sources reaches as far as several tens of meters, if light from a single source is split and then resynthesized, lights that has passed through an optical path difference that is shorter than the coherent length and then been synthesized causes strong interference, resulting in an even more prominent scintillation (i.e., interference fringes) than that from a high pressure mercury lamp.
Accordingly, the reduction of scintillation has become an essential technology, in particular, for the manufacturing of projectors that use laser light sources.
The following technologies have been disclosed as measures for reducing scintillation.
JP-A-11-038512 describes a screen that has a three layer structure made up of a diffusion layer, a transparent layer (i.e., a lenticular lens), and a diffusion layer, and in which the screen diffusion has been optimized. By making the scattering layers more complex in this manner, the randomness of the interference unevenness increases. As a result, if there is an increase in the number of fine components contained in the unevenness (i.e., interference fringes that have a small spatial frequency), an effect can arise that, when there is some movement in the line of sight, there is integral averaging of the light due to the residual image characteristics of the human eye. In particular, when viewing dynamic images, because there are frequent movements in the line of sight a reduction in scintillation can be anticipated.
JP-A-2001-100316 describes a screen in which light, an electric field, a magnetic field, heat, stress and the like are applied to a light scattering layer, and the shape, relative positional relationships, and index of refraction of the light scattering objects contained in this light scattering layer are changed over time. By changing the scatter distribution and phase of scattered waves from a light diffusion layer over time in this manner, the prevention of the occurrence of scintillation can be anticipated.
However, in JP-A-11-038512, because the scatter state of the final scattering surface is fixed, the spatial phase distribution between a viewer and a screen that is created by interference between light rays emitted from each point on the scattering surface is also fixed, and the interference unevenness is also visible as a fixed image. Accordingly, the interference unevenness does not completely disappear, and almost no effect can be obtained, particularly in projectors that are equipped with high interference laser light sources. Moreover, in a structure that is based on this type of high scattering, because there is a possibility that image blurring will be generated coincidentally, it is not possible to solve the original purpose of obtaining a high image quality.
Furthermore, in JP-A-2001-100316, a huge amount of driving energy is required in order to change the shape, relative positional relationships, index of refraction and the like of light scattering objects. Moreover, when these driving devices are used, the energy transmission efficiency to the scattering layer is poor, and there is a possibility that vibration, noise, unwanted electromagnetic waves, and exhaust heat will be generated and prevent a pleasant viewing experience. Furthermore, in a structure in which the scattering layer moves in the focusing direction, the size of the image changes. As a result, the positions of contour lines of an image also changes in a horizontal direction and becomes a factor in causing image blurring.